1. Field of the Invention
The present invention relates to a polarization direction synchronization detecting circuit and a receiving apparatus, and more particularly to a polarization direction synchronization detecting circuit and a receiving apparatus that detect, on a reception side, a light signal whose polarization direction is modulated on a transmission side that performs quantum cipher communication such as mobile communication of a satellite or the like, in synchronization with the transmission side.
2. Description of the Related Art
Concerning quantum cipher communication, I and others have proposed a technique of “a quantum cipher communication apparatus and method”, which is not published and is not a prior art (Japanese Patent Application Serial No. 2007-229604, filed on Sep. 5, 2007). The proposed technique is to provide first communicating unit which transmits and receives a communication signal formed by relatively strong pulse light between a transmitter and a receiver even when relative positions of the transmitter and the receiver change, and second communicating unit which transmits and receives a relatively weak quantum cipher signal in a period in which the first communicating unit is turned off.
Structures of the transmitter and the receiver included in the proposed quantum cipher communication apparatus are illustrated in FIGS. 4 and 5, respectively. In the transmitter illustrated in FIG. 4, first, a predetermined pattern (a PN signal) generated by a PN generator 52 according to a clock from a clock 51 is supplied, through a switch SW, to a communication laser 66 via a falling-edge trigger pulse generating circuit 58. The communication laser 66 generates a communication signal having wavelength λ1 of relatively strong pulse light and outputs the communication signal to a beam splitter 67. The beam splitter 67 directly reflects light having the wavelength λ1 and directly transmits weak light (a quantum cipher signal) having wavelength λ2 from a modulator 65.
A counter 56 counts clocks from the clock 51, and supplies a count value to a data controller 55. The data controller 55. controls the switch SW according to the count value. The switch SW is changed after the PN signal from the PN generator 52. Data (communication data) from an input data recorder 53 is supplied to the communication laser 66 through the falling edge trigger pulse generating circuit 58. When the falling-edge trigger pulse generating circuit 58 detects a filling edge of an input signal, the falling-edge trigger pulse generating circuit 58 drives first and second delay pulse generators 59 and 60 and the data controller 55. The data controller 55 causes, according to a random signal from a random generator 54, one of the first and second delay pulse generators 59 and 60 (depending on the output of the data controller 55) to generate a delay pulse after delay time D. A quantum cipher signal having the wavelength 2, which is weak compared with a communication signal, is generated from a first quantum laser 61 or a second quantum laser 62 according to the delay pulse and input to a polarization beam splitter 63. When polarization of the first quantum laser 61 is H and a polarization direction thereof is 0 degree, polarization of the second quantum laser 62 is V and a polarization direction thereof is 90 degrees. Each of the first quantum laser 61 and the second quantum laser 62 corresponds to 1 or 0 of a binary signal. The modulator 65 polarizes (modulates) a polarization direction of light output from the polarization beam splitter 63 by 45 degrees using the random signal from the random generator 54 as a modulation signal. The beam splitter 67 superimposes the output of the modulator 65 on the strong pulse light from the communication laser 66 and transmits the output.
In the receiver illustrated in FIG. 5, a light signal from the transmitter illustrated in FIG. 4 is distributed by a beam splitter 70. A strong pulse signal having the wavelength γ1 is converted into an electric signal by a communication receiver 71. One of the signals is supplied to a clock data recovery circuit 72. A falling edge signal of the other signal is supplied to a delay pulse generator 78 and output to first and second single photon receivers 76 and 77 as a gate signal after delay time. A clock and data output from the clock data recovery circuit 72 are input to a communication data recorder 73 and stored (recorded). The data is supplied to a matching detection circuit 79 together with the clock. When the data is compared with a PN signal (a predetermined data pattern transmitted prior to the data) from a PN generator 83 and coincidence of the data and the PN signal is detected, a counter 80 for counting clocks is reset according to detection output.
A light signal representing a weak coherent quantum cipher having the wavelength γ2 reflected by the beam splitter 70 is subjected to polarization (modulation) of 45 degrees by a signal from a random generator 84 in a modulator 74. Thereafter, a signal of polarized light H is separated to the first single photon receiver 76 and a signal of polarized light V is separated to the second single photon receiver 77 by the polarization beam splitter 75. The signals are respectively extracted by a gate signal from the delay pulse generator 78 and input to a data controller 81. In this way, at no signal time when transmission data falls on the transmission side, a quantum cipher signal transmitted at timing of fixed delay time from a falling edge can be extracted and recorded in the data controller 81.
FIG. 6 is a diagram of an example of quantum cipher communication by the transmitter and the receiver illustrated in FIGS. 4 and 5. When a satellite 92 is located over a ground station 90 at time T1, the satellite 92 generates, with a transmitter (having the structure illustrated in FIG. 4), a quantum key α and transmits a light signal superimposed on data A to the ground station 90. The ground station 90 receives the data A and the quantum key α with a receiver (having the structure illustrated in FIG. 5) and stores the data A and the quantum key α. Thereafter, when the satellite 92 moves and is located over a ground station 91 at time T2, the satellite 92 generates another quantum key β, superimposes the quantum key β on data B, and transmits the quantum key β to the ground station 91. The ground station 91 receives and stores the data B and the quantum key β. Further, the satellite 92 calculates exclusive OR of the quantum keys α and β to generate a quantum key γ, transmits the quantum key γ to the ground stations 90 and 91, and allows the ground stations 90 and 91 to share the quantum keys α and β by using the quantum key γ. In a case of FIG. 6, the same satellite 92 moves from a position on the ground station 90 to a position on the ground station 91. However, the satellite 92 may perform the quantum cipher communication with not-illustrated other satellites.
As explained above, polarization modulation is performed in synchronization with the quantum cipher signal from the transmitter and polarization is applied to two angles (θ1 and θ2) according to signals (“1” and “0”). In the example illustrated in FIG. 4, light polarized to 0 degree from the first quantum laser 61 (the polarized light H: in a case of θ1=0 degrees) or light polarized to 90 degrees from the second quantum laser 62 (the polarized light V: in a case of θ2=90 degrees) is selected. Polarization modulation is applied to the light and the light is transmitted.
An angle relation of polarization modulation between the transmission side and the reception side is illustrated in FIG. 7. In FIG. 7, θ1 and θ2 represent axes on the transmission side. This example is an example in which θ1 and θ2 are orthogonal (or may not be orthogonal). As illustrated when polarization modulation is performed on the transmission side with θ1 set to 0 degree and θ2 set to 90 degrees, the reception side of mobile communication relatively changes by an arrival angle Δθ (a phase difference Δθ) with respect to the transmission side. The horizontal direction in FIG. 7 represents a polarization base (direction) on the reception side.
FIG. 8 is a diagram of the intensity of light obtained by receiving a polarized and modulated signal through a polarizer. As illustrated in FIG. 7, the intensity is the intensity of light received through a polarizer that causes light obtained by polarizing the polarized and modulated signal to 0 degree to pass and does not cause light polarized to 90 degrees to pass. “A” represents a polarization modulation waveform of communication data signal A signal of 90 degrees and a signal of 0 degree are alternately generated on the transmission side. When the signal “A” is received at the phase difference Δθ=0 degree, even if the signal is received through the polarizer that causes the light polarized to 0 degree to pass, as illustrated in “B”, received light intensity of the light polarized to 0 degree is the maximum and received light intensity of the light polarized to 90 degrees is 0. On the other hand, when the phase difference is 0 degree <|Δθ|<45 degrees, even if the signal is received through the polarizer that causes the light polarized to 0 degree to pass, signal components of both the signal of 0 degree and the signal of 90 degree on the transmission side are received and a waveform represented by “C” is obtained. When the phase difference is 45 degrees <|Δθ|<135 degrees, the signal of 0 degree less easily passes through the polarizer. The received light intensity of the signal of 90 degrees is larger and a waveform represented by “D” is obtained. Phase is inverted from that of a transmission waveform in “A”. Further, when the phase difference is 135 degrees <|Δθ|<180 degrees, a signal waveform represented by “E” is obtained. The signal waveform is the same as that represented by “C”.
As explained above, a polarization angle relation of a light signal, an arrival angle relation of which relatively changes, changes as in a movable body (a transmission side or a reception side) such as a satellite. A synchronization detecting circuit for efficiently detecting an angle displacement component and adjusting a polarization axis direction on the reception side to a polarization base axis on the transmission side is necessary. However, the technique proposed by me and others of this application does not disclose a specific structure.
There is proposed a technique for creating a light wave, a polarization direction of which rotates in the detection of linearly-polarized light, transmitting the light wave through a polarization beam splitter to change light intensity, converting an electric signal with a photodetector, and performing synchronous detection with a signal synchronized with the rotation in the polarization direction using a lock-in amplifier (see Japanese Patent No. 2920502).
According to the technique disclosed in Japanese Patent No. 2920502, weak light is detected by rotating polarized light. Since the polarized light rotates on the reception side, the technique cannot be used for a quantum cipher signal used by fixing a polarization base axis.